1. Field of the Invention
The present invention relates to a distortion compensator, a transmitter, and a distortion compensation method used for, for example, a radio communication system and so on, and particularly relates to a distortion compensator, a transmitter, and a distortion compensation method using a distortion compensation technology of a power amplifier.
2. Description of the Related Art
Conventionally, a digital pre-distortion technique has been used as one of distortion compensation technologies of a power amplifier. An output signal of the power amplifier is fed back to a digital signal processor via an analog-digital converter (hereinafter, referred to as an “ADC”), and is compared with an input signal of the power amplifier, in a transmitter applying the digital pre-distortion technique. Distortion characteristics opposite to the distortion characteristics of the power amplifier are calculated based on the comparison result, and this opposite distortion characteristics are multiplied with the input signal of the power amplifier. A signal obtained by the multiplication is inputted to the power amplifier, and thereby, a distortion component included in the output signal of the power amplifier is canceled. (refer to Junxiong Deng, Prasad S. Gudem, Lawrence E. Larson, Donald F. Kimball, and Peter M. Asbeck, “A SiGe PA With Dual Dynamic Bias Control and Memoryless Digital Predistortion for WCDMA Handset Applications”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 41, NO. 5, pp. 1210-1221, MAY 2006)
In the conventional digital pre-distortion technique, a sampling rate of the ADC is always constant, and therefore, an A/D conversion is performed with a certain accuracy and sampling rate regardless of presence/absence of the distortion component in the output signal of the power amplifier. Accordingly, a constant calculation amount is always required in the distortion compensation process, and as a result, a constant power consumption, apparatus scale, countermeasure for heat generation and so on are always required.